“The protective buffer”: Adequate L4-S1 lordosis restoration reduces the symptomatic conversion of radiographic ASD and long-term revision rates — A 5-year multicenter cohort study

BACKGROUND: Inadequate restoration of L4-S1 segmental lordosis (SL) is a recognized contributor to adjacent segment disease (ASD). However, evidence regarding its impact on long-term revision rates remains conflicting, primarily due to short follow-up periods that fail to capture the critical transition from radiographic degeneration to symptomatic disease. PURPOSE: To evaluate the 5-year impact of L4-S1 SL restoration on revision rates and to introduce the concept of "Symptomatic Conversion Rate" to quantify the clinical progression of ASD. STUDY DESIGN: A multicenter, retrospective cohort study. PATIENT SAMPLE: A total of 232 patients who underwent L4-S1 transforaminal lumbar interbody fusion (TLIF) with a minimum 5-year follow-up were included. The cohort was stratified into Adequate and Inadequate Restoration groups based on postoperative L4-S1 SL. OUTCOME MEASURES: The primary endpoint was the 5-year revision surgery rate. Secondary endpoints included radiographic parameters (pelvic tilt [PT], pelvic incidence-lumbar lordosis [PI-LL] mismatch), implant failure rates, and patient-reported outcome measures (Oswestry Disability Index [ODI], Visual Analog Scale [VAS]). The "Symptomatic Conversion Rate" (the percentage of patients with radiographic ASD progressing to symptomatic ASD) was also assessed. METHODS: Clinical and radiographic data collected at a minimum of 5 years postoperatively were retrospectively analyzed. The cohorts were compared to assess the influence of L4-S1 SL restoration on long-term survivorship, functional recovery, and ASD progression. RESULTS: Baseline demographics and PI were comparable between groups (P > 0.05). At the 5-year follow-up, the Inadequate Group exhibited significant pelvic retroversion (PT increased to 21.82° vs. 17.26° in the Adequate Group, P < 0.001). Although radiographic ASD was prevalent in both cohorts (58.1% vs. 38.1%), the Symptomatic Conversion Rate was markedly higher in the Inadequate Group (73.3% vs. 28.1%, P < 0.001). Consequently, the 5-year revision rate was significantly greater in the Inadequate Group (26.4% vs. 2.4%, P < 0.001). The Inadequate Group also demonstrated inferior ODI scores (28.5 vs. 14.2, P < 0.001) and an increased incidence of screw loosening. CONCLUSIONS: Inadequate L4-S1 SL restoration is an independent risk factor for long-term revision surgery and implant failure following TLIF. Precise restoration provides a crucial "biomechanical buffer," preventing compensatory pelvic retroversion and significantly reducing the conversion of radiographic ASD into symptomatic disease. Achieving this protective buffer should be a primary surgical objective to maximize long-term construct survivorship.